Kim as many others play with the relaxation factors to avoid during the first iteration the warning! Personally, I do not like to reduce this relaxation factor, even more, usually I increase those parameter to obtain solucions in a shoter period of time.

Now, Kim suggests to adapt your mesh according to y+ and soon.... Hua, every case is different anybody can tell you where to adapt your grid without getting an idea of which type of geometry are you working with. Therefore, I strongly suggest that you check first where are you getting high turbulent viscosity ratio

Another point, Kim suggest that you run first the k-e model or one of its relatives, this suggestion is very important, I forgot to tell you because I think you have done this option before.

Now, come the question have you getting the same warning with k-e model??

Another point, Kim send you to use Quick discretization and soon... Careful, again this type of discretization is very sensitive. I strongly suggest that you try harder with the second order discretiztion before applying Kim's idea

Perhaps, You should know that overall what it is more important is that you provide a good estimation for the boundary condition (velocity, turbulent kinetic energy and dissipation rate) after all waht this codes made is just proyect your boundary condition into the domain.

My advice, Careful with the suggestion that you recieve, each person in this forum thinks that all the cases behave the same and that is not true

You are the only one who can evaluate if you keep going with this Kim' scheme, or you develop a systematic analysis of your case!!

Thanks you two, for your reply. I tried both of your ideas. I have changed to k-epsilon model first and adapt the grid base on Y+. The result seems converging, but there are still warnings that viscous ratio is too high in 80 cells, do I need to adapt the grid more, or can I trust this results? I check the FLUENT manual, not find the meaning of the turbulent viscosity ratio, can you please give me some explaination what it is and how it affect the results? Tnanks a lot.

Once again, you are too broad I don't know your case, your domain, your objectives, or even the project goal. Without this information it is impossible to provide an advise.

Hua, I know that the project is confidential and soon. However, You cannot be so general. For instante, you write it seems to converge. a case converge to a number in your case I don't know 1e-04???

The turbulence models selection depends of the complexity of the flow field and the need for accurate prediction, you can get in most of the cases convergence with the k-e model. However, you don't know if this model provide the most accurate flow field description until you compare with experiments or other source of data.

thanks, Alex. Actually I know the numerical result, if not compared to the experiment, can not be viewed as accurate or reliable. For some reasons, I can not talk about the detail of the problem, even inconvinient to do that since there is no figure-posting function here. For that run, what I can tell is that convergence is to 1E-5, but there is still warnnings about turbulent viscousity. So I am asking according to your experience, if this also happens in some of your work. And also if not troubling you too much, give us an introduction about the conception of this headache term--turbulence viscosity ratio. I believe I am not asking for specified case, just general explaination is fine. Thanks again, Alex.